I've been toying with the idea of designing swords that can be empowered by binding a soul into them, I'm just trying to get the requirements for the spell.
Here's a rough idea, I'll end up fiddling with it as I tweak it out.
Bind Soulblade [Advanced Spell of the Summon Sphere]
Power Point Cost: 6
Casting Time: Free Action, see below
Range: Touch, see below
Duration: Mortal
Saving Throw: Will Negates
Perquisites: Bind Spirit, Truename, Bless Weapon, Base Magic Attack +2, Base Attack Bonus +5 Knowledge Arcana 8 ranks, Knowledge Tactics 8 ranks
Magic Attack Roll: Sets save DC
This spell requires corruption check
This spell binds the soul of a living creature, outsider, elemental or other entity into an Akibitani weapon. The caster must know the Truneame of the target of the spell. The effect of this spell prevents an outsider from reforming and prevents a person from forming a ghost. If used on an undead it destroys the undead completely and it cannot reform or return through any means.
Once bound to a weapon the weapon gains a truename which the person casting the spell instantly knows. In order to wield the weapon the bearer must know the weapon's true name otherwise the weapon resists the user and deals 1d4 primal damage per round to the person trying to wield it.
The weapon gains certain abilities when bound:
Increased damage: The weapon deals an additional 1d4 damage per 3 HD (rounded down) of the entity bound into the weapon. This damage is considered normal damage if a person was bound, or primal if an outsider was bound.
Increased Hardness: The weapon's hardness increases by 1 per HD bound into it.
Increased Hit Points: The weapons HP is increased by the total HP of the entity bound into it.
Increased Attack Bonus: The weapon gains a enhancement bonus to attack equal to 1 per 4 HD (rounded down) of the bound entity.
Intelligent: The weapon gains mental stats of the entity bound into it. If rebound use whatever stats are higher (e.g. if you bind a Int 14, Wis 15, Cha 18 human, then a Int 23, Wis 16, Cha 16 Demon the weapon has stats of Int 23, Wis 16, Cha 18).
Ego: The item gains an Ego score equal to the number of HD bound into it. This means that should the bearer have equal or less levels than the item's ego score the item may attempt to compell the wielder up to once per day. The wielder must make a will save, the DC to resist is 10+ the item's Ego score. Failure results in the item Dominating the wielder (as per the spell). If the item's Ego is equal to or higher than the wielder's when this spell is complete the weapon can make a free domination attempt automatically without using up its limit of 1 attempt per day.
To cast this spell the caster must first have cast Bless Weapon on the weapon to be used as a focus, then must kill an with it. Upon killing the entity the caster casts Truename, the Bind Soulblade and the target makes a saving throw. If the save succedes the target resists the binding and the spell fails entirely. If the target fails the save their soul is bound into the weapon and the caster must make a corruption save DC = 10 +2 per time that the blade had been bound.
It is possible to rebind a blade, but doing so is dangerous and often times suicidal, although it has been done in order to gain massive power for a short time. If you requench a blade add together all HD bound into it to determine the new stats for the weapon. This almost always forces an ego challenge in such a situation, although it has been known to be used to bind powerful entities that couldn't be permanently killed simply to buy some time.